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.\" This documentation was written by Julien Ridoux at the University of
.\" Melbourne under sponsorship from the FreeBSD Foundation.
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.Dd December 1, 2011
.Dt FFCLOCK 4
.Os
.Sh NAME
.Nm FFCLOCK
.Nd Feed-forward system clock
.Sh SYNOPSIS
.Cd options FFCLOCK
.Sh DESCRIPTION
The
.Xr ntpd 8
daemon has been the dominant solution for system clock synchronisation for many
years, which has in turn influenced the design of the system clock.
The ntpd daemon implements a feedback control algorithm which has been
demonstrated to perform poorly in common use cases.
.Pp
Feed-forward clock synchronisation algorithms implemented by an appropriate
daemon, in concert with the
.Nm
kernel support, have been shown to provide highly robust and accurate clock
synchronisation.
In addition to time keeping, the
.Nm
kernel mechanism provides new timestamping capabilities and the ability to
use specialised clocks.
Feed-forward synchronisation is also very well suited for virtualised
environments, reducing the overhead of timekeeping in guests and ensuring
continued smooth operation of the system clock during guest live migration.
.Pp
The
.Nm
kernel support provides feed-forward timestamping functions within the kernel
and system calls to support feed-forward synchronisation daemons
.Po see
.Xr ffclock 2
.Pc .
.Ss Kernel Options
The following kernel configuration options are related to
.Nm :
.Pp
.Bl -tag -width ".Dv FFCLOCK" -compact
.It Dv FFCLOCK
Enable feed-forward clock support.
.El
.Ss Configuration
When feed-forward clock support is compiled into the kernel, multiple system
clocks become available to choose from.
System clock configuration is possible via the
.Va kern.sysclock
.Xr sysctl 8
tree which provides the following variables:
.Bl -tag -width "    " -offset indent
.It Va kern.sysclock.active
Name of the current active system clock which is serving time.
Set to one of the names in
.Va kern.sysclock.available
in order to change the default active system clock.
.It Va kern.sysclock.available
Lists the names of available system clocks
.Po
read-only
.Pc .
.El
.Pp
Feed-forward system clock configuration is possible via the
.Va kern.sysclock.ffclock
sysctl tree which provides the following variables:
.Bl -tag -width "    " -offset indent
.It Va kern.sysclock.ffclock.version
Feed-forward clock kernel version
.Po
read-only
.Pc .
.It Va kern.sysclock.ffclock.ffcounter_bypass
Use reliable hardware timecounter as the feed-forward counter.
Will eventually be useful for virtualised environment like
.Xr xen 4 ,
but currently does nothing.
.El
.Sh SEE ALSO
.Xr clock_gettime 2 ,
.Xr ffclock 2 ,
.Xr bpf 4 ,
.Xr timecounters 4 ,
.Xr sysctl 8
.Sh HISTORY
Feed-forward clock support first appeared in
.Fx 10.0 .
.Sh AUTHORS
.An -nosplit
The feed-forward clock support was written by
.An Julien Ridoux Aq Mt jridoux@unimelb.edu.au
in collaboration with
.An Darryl Veitch Aq Mt dveitch@unimelb.edu.au
at the University of Melbourne under sponsorship from the FreeBSD Foundation.
.Pp
This manual page was written by
.An Julien Ridoux Aq Mt jridoux@unimelb.edu.au
and
.An Lawrence Stewart Aq Mt lstewart@FreeBSD.org .
